THE MOST POWERFUL TELESCOPE. 341 



fine silver-on-glass reflector, with an aperture of eighteen inches and a 

 focal length of nearly ten feet. This telescope was mounted by him 

 on an equatorial stand of his own design, on what is known as the 

 " Sissons " principle. For efficiency, power, and excellence this eigh- 

 teen-inch reflector is as yet without a rival in England, and was only 

 beaten, perhaps, by the great refractor of the Washington Observatory. 

 With this instrument were made a number of observations of the faint 

 satellites of Saturn and Uranus, which rendered the Ealing Observa- 

 tory a familiar name to all astronomers. When, in 1877, the astro- 

 nomical world was electrified by the announcement of Professor Asaph 

 Hall's discovery of the two satellites of Mars, it was to Ealing that 

 astronomers looked for systematic observations of these faint objects, 

 and it was from Ealing Observatory that came the only systematic 

 series of raeasm-es of these objects which has been furnished by Eng- 

 land, Astronomers may congratulate themselves, therefore, upon this 

 new telescope being in good hands, and in an observatory where it will 

 not be allowed to rust in idleness like so many of the finest instruments 

 in England. 



Satisfied from the performance of his eighteen-inch Newtonian re- 

 flector that it would be possible to successfully construct much larger 

 instruments of this kind, it seems to have been about two years ago 

 that IVIr. Common first seriously thought of constructing a very large 

 reflecting telescope with a silver-on-glass speculum. It was obvious 

 that this would be a serious undertaking, and one which would require 

 much thought and ingenuity to carry it out successfully. Many diffi- 

 culties would require to be boldly faced and successfully overcome. 

 The expense alone would have been sufficient to deter most men. Ex- 

 perience, skill, courage, perseverance, money — all would be required if 

 success was to be won. 



It was decided to first undertake the manufacture of a telescope 

 with an aperture of thirty-seven and a half inches and a focal length 

 of about eighteen or twenty feet. This was a much shorter focus than 

 had usually been thought essential for an instrument of this large aper- 

 ture. Generally instruments of this kind are made with a focal length 

 of from nine to ten times their diameter. This would correspond to 

 about thirty feet focus for a speculum of the given size. The fine per- 

 formance of his eighteen-inch telescope had convinced Mr. Common 

 that it was not necessary to give a greater focal length than fifteen or 

 sixteen feet. But there were two conflicting interests to be reconciled. 

 The shorter the instniment the easier it would be to mount, and the 

 easier to observe with; but, on the other hand, the longer the focus 

 the better it would be for taking photographs of the heavenly bodies, 

 and this last was one of the main uses that the new telescope was in- 

 tended for. With the view of best reconciling these two views the 

 instrument was designed with a focus of some twenty feet. 



The very first step to be taken was to undertake the manufacture 



